Injection Device Comprising An Optical Sensor

ABSTRACT

The present invention relates to an injection device comprising an optically-based sensor for determining the position or setting of a dose setting member arranged to set a dose to be injected from the injection device. In particular, the present invention relates to an injection device comprising a rotatably mounted member having a plurality of optically coded paths arranged on an outer surface thereof. The rotatably mounted member is operatively connected to the dose setting member and is adapted to rotate with the dose setting member.

The present invention relates to an injection device comprising an optically-based sensor for determining the position or setting of a dose setting member arranged to set a dose to be injected from the injection device. In particular, the present invention relates to an injection device comprising a rotatably mounted member having a plurality of optically coded paths arranged on an outer surface thereof.

BACKGROUND OF THE INVENTION

WO 2005/004955 disclosed a pen-like injection device for administering a fluid product. The device comprises an apparatus for contactless measurement of a position between at least two elements that are movable relative to each other. The measurement apparatus encompasses at least two optical sensors that are placed across from each other in a stationary manner on at least one first element and a second element which is movable relative to said at least one first element. A surface profile is provided on the second element, said surface profile supplying a different predetermined profile pattern for each of the optical sensors when the first element and the second element are moved relative to each other.

Thus, according to the method suggested in WO 2005/004955 the optical sensors record different predetermined profile patterns along the surface profile when the first element is moved relative to the second element. The recorded data from the optical sensors are jointly processed in order to determine the relative position between the elements.

Thus, the apparatus for contactless measurements suggested in WO 2005/004955 applies the recoding of different predetermined profile patterns in order to determine the relative position of the two elements. The use of profiles makes the apparatus and method suggested in WO 2005/004955 rather complicated in that such profiles increase the dimensions, especially the diameter, of the pen-like device.

It is an objection of the present invention to provide an optically-based read-out mechanism for determining the position, relative or absolute position, of a dose setting member in a pen-like injection device.

SUMMARY OF THE INVENTION

The above-mentioned object is complied with by providing, in a first aspect, an injection device comprising a dose setting member adapted to set a dose of a medicament to be injected from the injection device, the injection device further comprising a rotatably mounted member having a plurality of optically coded paths arranged on an outer surface thereof. The rotatably mounted member may be operatively connected to the dose setting member and is adapted to rotate with the dose setting member.

Within the context of this application, an optical coded path is defined as a circular or helical shaped track defining a series of information elements spanning the length of the optical coded path. A plurality of optical coded paths may be arranged parallel to each other, thereby defining a matrix representing a two-dimensional matrix of information elements consisting of rows and columns. The optical coded paths may be arranged so as to adjoin neighbouring optical coded paths.

The optically coded paths may be provided by varying the optical properties along said paths, such as by varying the optical reflectivity or transparency of the outer surface of the rotatably mounted member. The optical reflectivity or transparency may be varied along the path by positioning strips of metal foil on the outer surface of the rotatably mounted member. Alternatively, the optically coding of the paths may be provided by printing non-reflecting regions on a piece of reflecting tape or other sticky material.

Preferably, six optically coded paths are arranged on the outer surface of the rotatably mounted member. Having six optically coded paths allows the angular resolution to be 1/64. Thus, the dose setting member can be positioned at 64 positions within one revolution of the dose setting member.

Alternatively, the six optically coded paths are arranged along a helically extending track around the rotatably mounted member. Thus, the six optically coded paths allows the matrix of information elements span the total length of a helically extending track. In some variants, the helical extending track extends less than 360 degrees around the rotatably mounted member. However, in preferred embodiments, the matrix of information elements extends along the helical track around more than one complete revolution of the rotatably mounted member. Having six optically coded paths, the information elements permits 64 distinct codes to be covered by the entire matrix. If a particular pen design permits the rotatably mounted member to be rotated in discrete steps of 15 degrees, and the pen design permits a total of 64 distinct allowed positions, then the matrix may be arranged to extend approximately 2.6 revolutions around the rotatably mounted member. Thus an absolute sensing scheme can be obtained, where each code corresponds exactly to one of the 64 discrete rotational positions that the rotatably mounted member may experience.

In a first embodiment of the first aspect of the present invention, the injection device may further comprise means for guiding a plurality of light beams, each light beam being guided so as to be incident on one optically coded path. Preferably, the number of guided light beams equals the number of optically coded paths. The plurality of light beams may be generated from a single light source, such as a LED or a laser diode. The light source may emit light in the visible or near infrared region. In order to provide a plurality of light beams the light emitted from the single lights source may be guided into separate channels which guide each of the beams towards an optically coded path.

The injection device may further comprise light detecting means having a plurality of light sensitive elements or areas. Preferably, the number of light sensitive elements or areas equals the number of optically coded paths. In order to guide light to the light sensitive elements the injection device may further comprise means for guiding a plurality of light beams reflected from the optically coded paths to the light sensitive elements. These light guiding means may be arranged as through-going holes/openings or channels in a housing.

The light guiding means for guiding light to the optically coded paths may be arranged so that the light beams are incident onto the optically coded paths under an angle of incidence in the range 40-50 degrees, such as under approximately 45 degrees. Similarly, the light guiding means for guiding light from the optically coded paths to the light sensitive elements may be arranged so that the light beams leave the optically coded paths under an angle of 40-50 degrees, such as under approximately 45 degrees.

In further aspects of the invention, the coding of the matrix elements may be arranged to define a Gray code coding scheme or a Johnson code coding scheme.

In a second embodiment of the first aspect of the present invention 13, the rotatably mounted member may be at least partly transparent. This embodiment of the aspect of the present invention may further comprise light emitting means adapted to couple a beam of light into the rotatably mounted member. This light emitting means may be a LED or a laser diode emitting light in the visible or in the near infrared region. Light detecting means having a plurality of light sensitive areas may also be included. The light detecting means may be arranged to detect light escaping the outer surface of the rotatably mounted member where the optically coded paths are arranged. Thus, the information applied in determining the position or setting of the dose setting member is based on light escaping the at least partly transparent rotatably mounted member which is operatively connected to the dose setting member. Preferably, the number of light sensitive areas equals the number of optically coded paths. Furthermore, the light sensitive areas are preferably arranged with the optically coded paths provided on the outer surface of the rotatably mounted member.

The injection device according to the first aspect of the present invention may further comprise an electronic control circuit operatively coupled to the light sensitive elements. The control circuit may process the electrical signals generated by the light sensitive elements in response to whether or not the light sensitive elements or areas receives reflected light from the highly reflective areas of the optically coded paths. By monitoring the intensity fluctuations of light reflected from the optically coded paths the position of the dose setting member may be determined. The control circuit may be in communication with a display means for displaying the determined setting of the dose setting member and/or display the set dose to be injected from the injection device.

In a second aspect, the present invention relates to a method for determining the position or setting of a dose setting member adapted to set a dose of a medicament to be injected from an injection device, the method comprising the steps of

-   -   projecting a plurality of light beams onto a plurality of         optically coded paths provided on an outer surface of a         rotatably mounted member, and     -   detecting light intensities of a plurality of light beams         reflected from the optically coded paths and determining the         position or setting of the dose setting member from the detected         intensities.

The rotatably mounted member is operatively connected to the dose setting member and is adapted to rotate with the dose setting member. Preferably, the number of light beams equals the number of optically coded paths provided on the rotatably mounted member.

The intensity of the plurality of optical light beams varies in accordance with the position of the dose setting member in that the reflectivity or transparency of the outer surface of the rotatably mounted member varies along the optically coded paths. Preferably, the number of light beams projected onto the outer surface of the rotatably mounted member and the number of light beams reflected thereof is equal to six.

The method according to the second aspect of the present invention may further comprise the step of providing information concerning the detected light intensities to a processing unit, said processing unit being adapted to calculate the position or setting of the dose setting member.

In a third aspect, the present invention relates to an arrangement for determining the position of a dose setting member adapted to set a dose of a medicament to be injected from an injection device, the arrangement comprising a rotatably mounted member having a plurality of optically coded paths arranged on an outer surface thereof, the arrangement further comprising means for generating and guiding a plurality of light beams towards the rotatably mounted member and means for guiding a plurality of light beams from the rotatably mounted member to a plurality of light sensitive elements or areas.

In a fourth aspect, the present invention relates to an arrangement for determining an amount of a medicament injected from an injection device, the arrangement comprising a rotatably mounted member having a plurality of optically coded paths arranged on an outer surface thereof, the arrangement further comprising means for generating and guiding a plurality of light beams towards the rotatably mounted member and means for guiding a plurality of light beams from the rotatably mounted member to a plurality of light sensitive elements or areas.

Thus, according to the fourth aspect of the present invention an arrangement for actually measuring an amount of medicament injected from an injection device is provided. The arrangement according to the fourth aspect may be implemented applying similar means as discussed in connection with the first aspect of the present invention. Thus, the optically coded paths may be provided by varying the optical properties along said paths, such as by varying the optical reflectivity or transparency of the outer surface of the rotatably mounted member. The optical reflectivity or transparency may be varied along the path by positioning strips of metal foil on the outer surface of the rotatably mounted member. Alternatively, the optically coding of the paths may be provided by printing non-reflecting regions on a piece of reflecting tape or other sticky material.

Preferably, the number of guided light beams equals the number of optically coded paths. The plurality of light beams may be generated from a single light source, such as a LED or a laser diode. The light source may emit light in the visible or near infrared region. In order to provide a plurality of light beams the light emitted from the single lights source may be guided into separate channels which guide each of the beams towards an optically coded path. Preferably, the number of light sensitive elements or areas equals the number of optically coded paths.

An electronic control circuit operatively coupled to the light sensitive elements or areas may further be provided. The control circuit may process the electrical signals generated by the light sensitive elements or areas in response to whether or not the light sensitive elements or areas receives reflected light from the highly reflective areas of the optically coded paths. By monitoring the intensity fluctuations of light reflected from the optically coded paths the position of the rotatably mounted member may be determined. The control circuit may be in communication with a display means for displaying an amount of medicament injected from the injection device. The control module may be arranged to display a dose just injected from the injection device and/or it may be arranged to display an accumulated dose injected from the injection device. By accumulated is meant the total volume or total amount of a series of doses injected from the injection device.

BRIEF DESCRIPTION OF THE INVENTION

The present invention will now be explained in further details with reference to the accompanying figures, wherein

FIG. 1 shows a first embodiment of the present invention,

FIG. 2 shows the first embodiment of the present invention wherein the rotatably mounted member is separated from the sensor housing,

FIG. 3 shows 6 optically coded paths arranged on the surface of the rotatably mounted member,

FIG. 4 shows a cross-sectional view of the first embodiment of the present invention,

FIG. 5 shows the optical path of a light beam in the first embodiment of the present invention, and

FIG. 6 shows a second embodiment of the present invention.

While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

In its most general aspect the present invention relates to an arrangement adapted for determining the position of a dose setting member of an injection device, such as a pen-type injection device. The medicament to be injected from the injection device can be insulin. The position of the dose setting member is determined by an optically based arrangement where light is either reflected from coded paths arranged on a surface of a rotatably mounted member or transmitted through the coded paths arranged on the surface of the rotatably mounted member.

A first embodiment of the present invention is depicted in FIG. 1 where a threaded, rotatably mounted member 1 is coupled to a sensor housing 2. The sensor housing 2 has a threaded inner surface adapted to cooperate and engage with the threaded outer surface of the rotatably mounted member 1. Thus, the sensor housing 2 and the rotatably mounted member 1 are coupled via a thread joint—see FIG. 2. The sensor housing 2 has a region 3 adapted to house one or more light sources and a region 4 adapted to house a number of light sensitive elements. Between the regions 3 and 4 a number of light guiding channels 5 exist. Thus, the one or more light sources, such as one or more LEDs or one or more laser diodes, emit light that is guided, via the light guiding channels 5, towards the outer surface of the rotatably mounted member 1. In the embodiment depicted in FIG. 1 the number of light guiding channels 5 is equal to 6. Thus, an LED positioned in region 3 generates 6 light beams which propagate towards the surface of the rotatably mounted member 1. The 6 light beams are incident under an angle of approximately 45 degrees.

The sensor housing 2 keeps the 6 light beams separated so as to avoid cross-talking between the beams. The separation of the light beams also applies to the receiver side of the sensor housing 2, i.e. to the section of the sensor housing 2 that guides light from the rotatably mounted member 1 and to the region 4 adapted to house a number of light sensitive elements. Thus, light beams approaching and leaving the optically coded paths are kept separated in order to avoid cross talk. In the region 4 a photo detector is aligned with each of the light guiding channels guiding light from the rotatably mounted member 1. Thus, in the embodiment depicted in FIG. 1 a sensor array having 6 photosensitive areas is applied. Alternatively, 6 individual photo detectors can be positioned in the region 4.

At the intersection between each of the 6 light beams generated by the light source and the outer surface of the rotatably mounted member an optically coded path is positioned. This optically coded path is a well-defined pattern of optically reflecting and optically absorbing areas arranged along a string or track. The optically coded paths are arranged in the valleys of the rotatably mounted member 1—i.e. in the region denoted 6 in FIG. 2.

Evidently, the optically coded paths have a helical shape in that they follow the region 6 of the rotatably mounted member 1. This also implies that the lengths of the optically coded paths exceed a distance longer than the circumference of the rotatably mounted member 1. A rotatably mounted member having 6 optically coded paths arranged thereon is illustrated in FIG. 3. Each path has a width of approximately 1 mm. By having 6 optically coded paths the number of well-defined positions along the length of the entire path equals 64 (2⁶=64).

The valleys referred to above corresponds to a helically extending recessed track wherein the optical paths are arranged in parallel in the bottom of the recessed track.

An optically coded path is implemented by positioning an aluminium tape on the rotatably mounted member, and positioning, on the aluminium tape, a transparent tape having black or grey areas printed thereon. The black or grey regions act as absorbing areas of the path whereas the aluminium areas (visible through the transparent tape) act as the reflecting areas of the path.

FIG. 4 shows a cross-sectional view of the rotatably mounted member 1 and the sensor housing 2. As seen the sensor housing 2 abuts the region 6 of the rotatably mounted member 6. The sensor housing 2 is fixed in relation to the injection device whereas the rotatably mounted member 1 is adapted to rotate relative to the injection device when a dose of medicament is to be set. Thus, when setting a dose to be injected the optically coded paths arranged in the region 6 of the rotatably mounted member 1 will pass by an intersection zone where the 6 light beams from the light source hit the rotatably mounted member. This relative movement between the sensor housing 2 and the optically coded paths on the rotatably mounted member 1 will induce intensity fluctuations of the light beams reflected from the paths. For example, if one of the incident light beams hits a highly reflective area a high intensity signal is reflected, whereas if the same incident light beam at another position of the rotatably mounted member 1 relative to the sensor housing 2 hits an absorbing area essentially no signal is reflected. By constantly measuring the reflected signals from all 6 optical paths the combined angular and axial position of the rotatably mounted member, and thereby the position of the dose setting member, can be determined.

The beam path from the light source to the photo detector is depicted in FIG. 5. The light source is positioned in region 3 and light beams propagate in channels 5 toward a detector positioned in region 4. Firstly, light beams generated by the light source propagate in the horizontal part of the channels 5. The beams are redirected into the vertical part of the channels 5 so as to hit the detector in region 4. The redirection of the beams is caused by an interaction of the beams with the surface of the rotatably mounted member 1.

FIG. 6 shows a second embodiment of the present invention. In this second embodiment the rotatably mounted member is transparent in that it must be capable of guiding light coupled into it. As illustrated in FIG. 6 light from an LED is coupled into the rotatably mounted member where it is guided to the region where optically coded paths are arranged on the surface of the rotatably mounted member. The optically coded paths are arranged by positioning a well-defined pattern of light absorbing elements on the surface of the rotatably mounted member so that light propagating inside the rotatably mounted member is only allowed to escape from the transparent, rotatably mounted member in certain regions along the optically coded paths. The optically coded paths can be arranged as illustrated in FIG. 3. A sensor array having a plurality of light sensitive elements is positioned above the optically coded path so that light escaping the rotatably mounted member can be detected. By detecting the intensity variations caused by a relative movement of the sensor array and the rotatably mounted member the position of this member, and thereby the position of the dose setting member, can be determined.

As already mentioned the 6 optically coded paths allow 64 positions of the dose setting member. Each of these positions correspond a certain amount of medicament, such as insulin. Thus, when the user of the injection device sets a dose to be injected, the user positions the dose setting member at the position that corresponds to the amount of medicament to be injected from the injection device.

The number of optically coded paths arranged on the rotatably mounted member may also be different from 6. For example if the number of well-defined positions required exceeds 64 more than 6 optically coded paths are needed. If only the relative position of the dose setting member is required only 2 optically coded paths are required.

The light source may in principle be any light capable of fitting into the region 3. Obvious choices would be LEDs or laser diodes emitting light in the visible or near infrared regions. The detector for detecting light reflected from optically coded paths can be sensor arrays comprising a plurality light sensitive regions or individual photo detectors. The number of light sensitive areas on a sensor array or the number of individual sensors matches the number of light guiding channels arranged to guide light from the rotatably mounted member and to the detector(s). Similarly, the number of light guiding channels guiding light from the light source and the rotatably mounted member equals the number of light guiding channels arranged to guide light from the rotatably mounted member and to the detector(s). 

1. An injection device comprising: a dose setting member adapted to set a dose of a medicament to be injected from the injection device, and a rotatably mounted member having a plurality of optically coded paths arranged on an outer surface thereof.
 2. The injection device according to claim 1, wherein the optically coded paths are provided by varying the optical properties along said paths.
 3. The injection device according to claim 2, wherein the optical properties along the paths are varied by varying an optical reflectivity or transparency of the outer surface of the rotatably mounted member.
 4. The injection device according to any of claim 1 wherein six optically coded paths are arranged on the outer surface of the rotatably mounted member.
 5. The injection device according to claim 1 further comprising structure for guiding a plurality of guided light beams, each light beam being guided so as to be incident on one optically coded path.
 6. The injection device according to claim 5, wherein the number of guided light beams equals the number of optically coded paths.
 7. The injection device according to claim 5, further comprising a light detector, the light detector comprising a plurality of light sensitive elements.
 8. The injection device according to claim 7, wherein the number of light sensitive elements equals the number of optically coded paths.
 9. The injection device according to claim 7, further comprising structure for guiding a plurality of light beams reflected from the optically coded paths to the light sensitive elements.
 10. The injection device according to claim 5, wherein the light guiding structure arranged to provide light beams to and from the optically coded paths are arranged as through-going openings in a housing.
 11. The injection device according to claim 10, wherein the light guiding structure for guiding light to the optically coded paths are arranged so that the light beams are incident onto the optically coded paths under an angle of 40-50 degrees.
 12. The injection device according to claim 10, wherein the light guiding structure for guiding light from the optically coded paths to the light sensitive elements are arranged so that the light beams leave the optically coded paths under an angle of 40-50 degrees.
 13. The injection device according to claim 1, wherein the rotatably mounted member is at least partly transparent.
 14. The injection device according to claim 13, further comprising a light emitter adapted to couple a beam of light into the rotatably mounted member.
 15. The injection device according to claim 14, further comprising light detecting structure having a plurality of light sensitive areas, the light detecting structure being arranged to detect light escaping the outer surface of the rotatably mounted member.
 16. An injection device according to claim 15, wherein the number of light sensitive areas equals the number of optically coded paths.
 17. A method for determining the position or setting of a dose setting member adapted to set a dose of a medicament to be injected from an injection device comprising a dose setting member, the method comprising: projecting a plurality of light beams onto a plurality of optically coded paths provided on an outer surface of a rotatably mounted member, and detecting light intensities of a plurality of light beams reflected from the optically coded paths, and determining the position or setting of the dose setting member from the detected intensities.
 18. The method according to claim 17, wherein the number of light beams equals the number of optically coded paths provided on the rotatably mounted member.
 19. The method according to claim 17, wherein the intensity of the plurality of optical light beams varies in accordance with the position of the dose setting member in that the reflectivity or transparency of the outer surface of the rotatably mounted member varies along the optically coded paths.
 20. The method according to claim 17, wherein the number of light beams projected onto the outer surface of the rotatably mounted member and the number of light beams reflected thereof is equal to six.
 21. The method according to claim 17, further comprising of providing information concerning the detected light intensities to a processing unit, said processing unit being adapted to calculate the position or setting of the dose setting member.
 22. An arrangement for determining the position of a dose setting member adapted to set a dose of a medicament to be injected from an injection device, the arrangement comprising: a rotatably mounted member having a plurality of optically coded paths arranged on an outer surface thereof, a light generator for generating and guiding a plurality of light beams towards the rotatably mounted member, and structure for guiding a plurality of light beams from the rotatably mounted member to a plurality of light sensitive elements or areas.
 23. An arrangement for determining an amount of a medicament injected from an injection device, the arrangement comprising: a rotatably mounted member having a plurality of optically coded paths arranged on an outer surface thereof, a light generator for generating and guiding a plurality of light beams towards the rotatably mounted member, and structure for guiding a plurality of light beams from the rotatably mounted member to a plurality of light sensitive elements or areas.
 24. The arrangement according to claim 22, wherein the optically coded paths are provided by varying the optical properties along said paths.
 25. The arrangement according to claim 24, wherein the optical properties along the paths are varied by varying an optical reflectivity or transparency of the outer surface of the rotatably mounted member.
 26. The arrangement according to any of claim 22, wherein six optically coded paths are arranged on the outer surface of the rotatably mounted member.
 27. The arrangement according to claim 22, further comprising structure for guiding a plurality of guided light beams, each light beam being guided so as to be incident on one optically coded path.
 28. The arrangement according to claim 27, wherein the number of guided light beams equals the number of optically coded paths.
 29. The arrangement according to claim 27, further comprising a light detector, the light detector comprising a plurality of light sensitive elements.
 30. The arrangement according to claim 29, wherein the number of light sensitive elements equals the number of optically coded paths.
 31. The arrangement according to claim 29, further comprising structure for guiding a plurality of light beams reflected from the optically coded paths to the light sensitive elements.
 32. The arrangement according to claim 27, wherein the light guiding structure arranged to provide light beams to and from the optically coded paths are arranged as through-going openings in a housing.
 33. The arrangement according to claim 32, wherein the light guiding structure for guiding light to the optically coded paths are arranged so that the light beams are incident onto the optically coded paths under an angle of 40-50 degrees.
 34. The arrangement according to claim 32, wherein the light guiding structure for guiding light from the optically coded paths to the light sensitive elements are arranged so that the light beams leave the optically coded paths under an angle of 40-50 degrees.
 35. The arrangement according to claim 22, wherein the rotatably mounted member is at least partly transparent.
 36. The arrangement according to claim 35, further comprising a light emitter adapted to couple a beam of light into the rotatably mounted member.
 37. The arrangement according to claim 36, further comprising light detecting structure having a plurality of light sensitive areas, the light detecting structure being arranged to detect light escaping the outer surface of the rotatably mounted member.
 38. The arrangement according to claim 37, wherein the number of light sensitive areas equals the number of optically coded paths.
 39. The arrangement according to claim 23, wherein the optically coded paths are provided by varying the optical properties along said paths.
 40. The arrangement according to claim 39, wherein the optical properties along the paths are varied by varying an optical reflectivity or transparency of the outer surface of the rotatably mounted member.
 41. The arrangement according to any of claim 23, wherein six optically coded paths are arranged on the outer surface of the rotatably mounted member.
 42. The arrangement according to claim 23, further comprising structure for guiding a plurality of guided light beams, each light beam being guided so as to be incident on one optically coded path.
 43. The arrangement according to claim 42, wherein the number of guided light beams equals the number of optically coded paths.
 44. The arrangement according to claim 42, further comprising a light detector, the light detector comprising a plurality of light sensitive elements.
 45. The arrangement according to claim 44, wherein the number of light sensitive elements equals the number of optically coded paths.
 46. The arrangement according to claim 44, further comprising structure for guiding a plurality of light beams reflected from the optically coded paths to the light sensitive elements.
 47. The arrangement according to claim 42, wherein the light guiding structure arranged to provide light beams to and from the optically coded paths are arranged as through-going openings in a housing.
 48. The arrangement according to claim 47, wherein the light guiding structure for guiding light to the optically coded paths are arranged so that the light beams are incident onto the optically coded paths under an angle of 40-50 degrees.
 49. The arrangement according to claim 47, wherein the light guiding structure for guiding light from the optically coded paths to the light sensitive elements are arranged so that the light beams leave the optically coded paths under an angle of 40-50 degrees.
 50. The arrangement according to claim 23, wherein the rotatably mounted member is at least partly transparent.
 51. The arrangement according to claim 50, further comprising a light emitter adapted to couple a beam of light into the rotatably mounted member.
 52. The arrangement according to claim 51, further comprising light detecting structure having a plurality of light sensitive areas, the light detecting structure being arranged to detect light escaping the outer surface of the rotatably mounted member.
 53. The arrangement according to claim 52, wherein the number of light sensitive areas equals the number of optically coded paths. 